In recent years, microparticles are used widely in various industrial fields such as optical materials, magnetic materials, electrical conductive materials, electronic materials, functional ceramics, fluorescent materials, catalyst materials, chemical material, and so forth. In accordance with the increasing requirements in goods for multifunction, miniaturization, and the like, problems to be solved is to stuff functions thereinto as much as possible, and in addition, to reduce the size and the weight thereof further down than now; and in order to fulfill these needs, microparticles are considered to be indispensable. In addition, by making microparticles, the particles may acquire new physical properties such as enhanced activity and transparency. However, for example, in the case of barium titanate or the like that is used for a derivative thin film, it is known that there is a close relationship between the crystallite diameter and the characteristics of microparticles, as shown by the example that an intended physical properties cannot be obtained if the crystallite diameter in the microparticles becomes too small. Accordingly, it, is considered that microparticles need to be control not only in the particle diameter thereof but also in the crystallite diameter thereof.
Generally, the crystallite means the maximum congregate that can be considered to be a single crystal; and the size of this crystallite is called as the crystallite diameter. To measure the crystallite diameter, there are a method that lattice fringe of the crystallite is confirmed by using an electron microscope and a method that the crystallite diameter is calculated from the diffraction pattern obtained by using an X-ray diffraction apparatus and the Scherrer equation.Crystallite diameter D=K×λ/(β×cos θ)  Scherrer equation
Here, for calculation, K, the Scherrer's constant, is K=0.9; λ is the wavelength of the X-ray tube used; β is the half-width; and θ is the diffraction angle.
Illustrative example of the method for controlling the crystallite diameter of microparticles includes: a method wherein a metal single body, a metal ion, a metal compound, or a metal solution having these entities dissolved in a solvent is treated by the solvothermal method as shown in Patent Document 1; and a method wherein a hydrothermal treatment is conducted under a subcritical or a supercritical condition, or a heat treatment is conducted under an inert atmosphere, as shown in Patent Documents 2 to 4. In these methods, an apparatus having superior heat resistance and pressure resistance as well as an inert atmospheric condition is required, and moreover time is necessary for these treatments; and thus, there are problems of increase in an energy cost and so forth.
The applicant of the present invention provided the method for producing microparticles shown in Patent Document 5, in which control of the particle diameter was disclosed; however, there has been no disclosure as to a method for controlling the crystallite diameter.